Avionic displays require high reliability. Avionic displays, such as, active matrix liquid crystal displays (AMLCD), are utilized in military and commercial aircraft to present navigational, flight, and target information. Avionic displays typically require a back light source that is capable of producing wide dynamic luminance ranges, of providing long operational life, and of operating over a temperature range of -20.degree. celsius (C.) to +90.degree. (C.). The back light must produce appropriate luminance over diverse operating conditions, such as, in direct sunlight, at night, and in fog conditions.
As is known in the art, specialized fluorescent lamps are utilized to produce the luminescence in accordance with the high performance specifications of AMLCDs. These specialized fluorescent lamps used in avionic back-lighting applications can cost almost fifty times as much as commercially available fluorescent lamps for ordinary consumer applications. The specialized fluorescent lamp can be fabricated from a four-foot length of sodium-lime glass tubing. The lamp is hand-crafted into a five-bend serpentine lamp configuration. This configuration is necessary to obtain sufficient phosphor area for the demanding luminance requirements associated with readability of avionic displays over various lighting conditions. The fabrication of the specialized fluorescent lamp is labor-intensive and extremely expensive.
Additionally, the fluorescent lamp is often bonded to a lamp heat sink to ensure lamp reliability. Allowing the lamp to become over heated seriously jeopardizes the dependable operation of the lamp and reduces its operating life. The lamp is permanently attached to heat sink with a two-part thermally conductive silicon elastomer adhesive system.
Lamp heat sinks are necessary because back light sources for avionic displays must be passively cooled and yet be operational over a large temperature range. Therefore, lamp reliability is especially critical since lamp replacement is not economically feasible once the lamp is permanently bonded to the lamp heat sink.
The lamp and lamp heat sink are generally assembled within a rear modular chassis for top level assembly into the aircraft or other system. The top level assembly additionally makes lamp replacement impracticable due to disassembly labor costs.
Specialized conventional fluorescent lamps include a pair of filaments or cathode coils disposed at opposite ends of glass tubing. The glass tubing has an interior surface coated with a phosphor material and is filled with a gas mixture of argon and mercury. The cathode coils emit ions which travel from the cathode coil through a cathode fall region and to a positive column. The gas mixture produces a characteristic light blue glow throughout the lamp when the ions travel through it.
However, if the lamp driver or inverter produces insufficient output current and the filaments (e.g., cathodes) are not operating at sufficient temperature (e.g. too cold), the filaments do not produce enough thermionic emissions, and mercury ions can bombard the filaments. The bombardment of the filaments or cathode coils can sputter away the cathode oxide coating on the coils, thereby severely reducing the operating lifetime of the lamp. Heretofore, prior art fluorescent lamps and avionic displays have not provided any warning or indication of the deterioration of the cathode oxide coating. As stated above, this is particularly problematic because lamps are particularly difficult to replace once they have been installed in an avionic system.
Thus, there is a need for a lamp or light which provides an indicator of reduced lamp operating life. Further still, there is a need for an integral cathode fall indicator which indicates a voltage across a cathode fall region, thereby indicating proper operation of the fluorescent lamp.